The 10-year-old twin boys are prize fighters in the making, and they waste no opportunity to duke it out.

In between pounding a punch bag and kicking a pad held by a middle-aged trainer, Pornsak and Jirasak Katsriphuak clamber up into the ring and let fly at each other.

The diminutive wannabe Muay Thai bruisers each weigh just over 20kg. During their sparring bouts they pull few of their punches, bobbing and weaving in imitation of professional boxers before unleashing flurries of kicks and punches.

Jirasak tends to get the upper hand, but his brother will not back down. Pornsak weathers an onslaught of flailing fists and flying feet from Jirasak, before sending his twin tumbling by sweeping a leg out from under him. In a refereed fight, Pornsak would have earned points for the move.

Then he gets caught off guard as Jirasak lands a well-timed kick on his head. Wincing with pain, Pornsak drops to the floor to nurse his ear. In a moment, though, he is back on his feet for another round of rough and tumble.

“They’re like angry grasshoppers. They hop and skip a lot,” says Jamlong Jaipakdee, 49, who helps train the boys at a spartan, outdoor boxing camp set up between two concrete pylons underneath an elevated highway by the side of a busy road in central Bangkok. The brothers train alongside fighters twice their age during intense sessions where sinewy bodies glisten with sweat in the clammy tropical heat.

“I can see them both going far in this sport,” says Jamlong, a heavily tattooed former professional fighter. “They have plenty of fire in the belly.”

It isn’t only in sparring sessions that the two youngsters exchange blows. The twins, who started training in Thai boxing at age six, each have a dozen fights under their belt. They have fought and beaten children their own age in Bangkok and the countryside, with some tournaments ending in a knockout.

And just like fights with older boxers, those for boys are lively affairs. Parents and spectators egg young contestants on from ringside, rewarding each punch, knee thrust, high kick and elbow strike with an enthusiastic “Ahay!” “Oy!” or “Aye!” There is also a good deal of betting going on as inveterate gamblers place wagers on the budding pugilists. Plenty of cash can change hands during these junior contests.

“I like punching and kicking,” Jirasak says. “I especially like using the elbows. You can knock out your opponent by elbowing him in the face.”

Jirasak is earnest, his solemn mien in contrast to his brother’s more genial disposition. Jirasak lost a recent fight when a knee to his abdomen sent him to the floor, where he lay doubled up in pain. “It hurt,” he recalls.

His brother had his mouth bloodied in his own fight. “I didn’t cry,” Pornsak says. Their mother, La-ong Katsriphuak, wasn’t pleased. “I don’t want to see them get hurt,” La-ong, a street vendor, says. “But they want to fight and I support them.”

Their father is more sanguine. “In the ring it’s them against an opponent. Only they can help themselves,” says Somsak Katsriphuak, 42, a handyman who was once a boxer himself. “They win or lose based on their own merits.”

I like punching and kicking. I especially like using the elbows. You can knock out your opponent by elbowing him in the face

Jirasak Katsriphuak, 10-year-old Muay Thai boxer

Win or lose, his sons earn 300 baht (US$9) a fight – a paltry sum, even by local standards. Yet the prospect of far more cash beckons once they can turn professional at the age of 15.

For underprivileged children such as Pornsak and Jirasak, a career in Muay Thai can be a ticket to fame and fortune in a boxing-obsessed nation where champion fighters are feted as celebrities. The twins live in an inner-city community of modest dwellings along a fetid canal. Five days a week, after school, they go to training. Their next fights will be in a few weeks’ time and they can barely wait.

Critics say encouraging children to fight in the ring is a form of child abuse. Several physicians in Thailand have been warning that young boxers can suffer lasting brain damage from knocks to the head.

“Brain injuries caused by boxing can have long-term effects on the neurological system [of children],” explains Professor Jiraporn Laothamatas, a neuro-radiologist at Ramathibodi Hospital in Bangkok.

After conducting tests on 300 primary-school-age boxers, Jiraporn’s team found that many had markedly lower IQs than average. The longer they had been fighting, the lower their IQ scores were. Unless they can earn enough as professional fighters to set themselves up for life, these children are likely to have limited prospects.

“How can they possibly study and find [well-paying] jobs?” Jiraporn says. “What will their quality of life be with a bruised brain and low IQ?”

Pumpichai Rattanawisit, a trainer and fight promoter, makes light of such concerns. “Kids can get hurt,” he concedes. “But their injuries aren’t serious. A bruise here and there. Children aren’t strong enough to injure each other seriously.”

Pumpichai, 60, runs a decrepit open-air gym in a squalid Bangkok slum where, on a recent afternoon, five boys in their early teens pummelled tatty punchbags, lifted rusty weights and skipped rope. Outside, chickens rooted through mounds of rotting garbage by a scummy stream of putrid water.

“These kids can make 2,000 baht a fight,” says the promoter, who gets a cut from their earnings. “They’re happy with that. They come from poor families.”

One of his protégés is Teerapong Khantong, a fresh-faced 12-year-old who has already had 71 fights. He has won 60 or so of them.

“I like fighting. It’s easy to make money this way,” he says. “I give what I earn to my parents.”

His father is a security guard, his mother is a housekeeper. “My mother is concerned about me when I fight,” Teerapong says. “I tell her not to worry.”

The gym has been operating for 40 years. Several alumni have become celebrated champions at the Lumpini and Rajadamnern boxing stadiums, Thailand’s two most prestigious Muay Thai venues. Fading news clippings of them are pinned up on vertically mounted strips of cardboard.

“I motivate the kids by telling them, ‘Do you want to be like these guys in the pictures? Do you want to be champions and make lots of money?’” Pumpichai says.

Anywhere between 200,000 and 300,000 children around Thailand are believed to be training in Muay Thai, and lots of them want to become champions. Many of them start competing in the ring as young as age four, often without protective gear such as a head guard.

Pacharadanai Poolsawat was one such youngster. A mild-mannered 20-year-old whose entire back and left front is covered in picaresque yakuza-style tattoos, Pacharadanai had his first fight when he was four in his hometown of Khorat, in northeastern Thailand. “The other kid was seven or eight, much older than me. He kneed me in the side,” he says. “I cried a lot, but mostly because I lost.”

These days, Pacharadanai rarely loses a fight. He is the flyweight champion at Rajadamnern Stadium, Bangkok’s oldest boxing venue.

Pacharadanai, who has had more than 200 fights so far, can earn up to 540,000 baht per fight – three times the average annual wages in Thailand. A picture of him from a magazine, displayed at the alfresco boxing camp where the twins train, shows him knocking out a Japanese opponent.

“I want to be a fighter like him,” Pornsak says sheepishly.

The twins are on the right track for that, Pacharadanai notes. Muay Thai is a highly competitive sport, and the sooner they start training and getting some fighting experience in the ring, the better.

“One and Two are at the best age,” says Uthai Siriyota, an experienced trainer who helps out at the training camp. He is referring to the twins by the nicknames bestowed on them lightheartedly by older fighters. “Children their age can easily learn all the techniques,” Uthai, 48, adds. “They can also learn to take it on the chin in the ring.”